r6839: Add support for building subsystems as shared libraries. This can be

r6839: Add support for building subsystems as shared libraries. This can be
done by setting:
OUTPUT_TYPE = SHARED_LIBRARY
in the [SUBSYSTEM::...] section belonging to a subsystem.

The idea is to allow multiple values to OUTPUT_TYPE simultaneously
(e.g. OUTPUT_TYPE = SHARED_LIBRARY, STATIC_LIBRARY, OBJLIST )
(This used to be commit b9d0ae93ba86fec0115f58e7940b2a6c908bc809)